This application makes reference to and claims all benefits accruing under 35 U.S.C. Section 119 from an application entitled xe2x80x9cDistributed Feedback Semiconductor Laserxe2x80x9d filed in the Korean Industrial Property Office on Oct. 12, 2001 and assigned Serial No. 2001-62881, the contents of which are hereby incorporated by reference.
1. Field of the Invention
The present invention relates generally to semiconductor lasers. More particularly, the present invention relates to a distributed feedback semiconductor laser.
2. Description of the Related Art
In general, a distributed feedback semiconductor laser is formed on a semiconductor substrate so as to have a guide layer that serves as a light traveling path. Gratings are formed on the lower surface of the guide layer.
FIG. 1 is a sectional view of a conventional distributed feedback semiconductor laser. Referring to FIG. 1, the distributed feedback semiconductor laser includes a semiconductor substrate 110, first and second clad layers 120 and 160, a guide layer 140, an active layer 150, first and second upper electrodes 170 and 180, a lower electrode 190, and first and second reflective layers 200 and 210. The distributed feedback semiconductor laser is divided into a first laser oscillation section 230 and a second laser oscillation section 240 with respect to a central line 220.
The first clad layer 120 has gratings 130 on its upper surface. The gratings 130 have a predetermined period and a distributed feedback wavelength is determined by the grating period. The active layer 150 is formed in a multiple quantum well structure and the distributed feedback semiconductor laser has a 2-electrode structure. During DC operation, when predetermined currents I1 and I2 are applied to the first and second upper electrodes 170 and 180, respectively, a predetermined gain is achieved from the active layer 150, and light with a distributed feedback wavelength is oscillated as in a conventional 1-electrode distributed feedback semiconductor laser. During AC operation, when the predetermined current I1 and a modulated current (I1+xcex94I2) are applied to the first and second upper electrodes 170 and 180, respectively, a gain decrease at a low current level is compensated for by a gain generated by application of the current I1, thus obtaining a derivative gain at or above a predetermined level.
FIG. 2 is a carrier density-gain graph in a distributed feedback semiconductor laser. FIG. 2 graphically illustrates that the gain increases with a carrier density, and a derivative gain, which is defined as a ratio of a fine carrier density increment to a fine gain increment, decreases with the carrier density.
However, the conventional distributed feedback semiconductor laser experiences a great loss of light in the guide layer 140 and the active layer 150. Therefore, about 1018[cmxe2x88x923] or more carriers are required in the active layer 150 in order to oscillate the distributed feedback semiconductor laser. Meanwhile, as the carrier density increases, the derivative gain decreases and the amount of light loss increases. In FIG. 2, N2 denotes the carrier density of the active layer 150 and a derivative gain-carrier density curve at N2 is illustrated. If the level of the input current is increased to compensate for the decrease of the derivative gain, a non-radiative recombination rate increases. The resulting heat emission deteriorates the temperature characteristics of the distributed feedback semiconductor laser, making its operation at high temperature impossible.
Moreover, high modulation of the distributed feedback semiconductor laser by a high-level current worsens chirping due to carrier fluctuation and thus, limits the frequency bandwidth. Consequently, the transmission characteristics of the distributed feedback semiconductor laser deteriorate.
It is, therefore, an object of the present invention to provide a distributed feedback semiconductor laser that maximizes the derivative gain.
The above and other objects are achieved by providing a distributed feedback semiconductor laser. In the distributed feedback semiconductor laser, first and second clad layers having predetermined refractive indexes are formed on a semiconductor substrate. A guide layer propagates light between the first and second clad layers. An oscillating clad layer oscillates light at a predetermined wavelength and an amplifying clad layer amplifies the light with a predetermined gain between the first clad layer and the guide layer. The distributed feedback semiconductor laser is divided into a laser oscillation section including the oscillating clad layer and a laser amplification section including the amplifying active layer. First and second gratings are formed on the lower surface of the guide layer in the laser oscillation section and in the laser amplification section, respectively.